Facsimiles are well known for exchanging written time-critical information. Early facsimile standards, i.e., CCITT Group 1 (1960) and CCITT Group 2 (1976) established a relatively slow rate of transmitting data between terminals with times of 5 or 6 minutes per page being common. These standards are used rarely today but are still supported for backward compatibility. The current dominant standard is CCITT Group 3 which has boosted image quality and reduced transmission time to as little as 6 seconds per page. Group 1 and Group 2 standards use tonal signal for control and handshaking whereas Group 3 uses binary coded Frequency Shift Keying (FSK) signalling.
Modems are also used extensively for transmitting data over the public switched telephone network between terminals including computers and the like.
Facsimile transmission between terminals is half-duplex, meaning that only one of the two terminals is transmitting at any one time. During a typical facsimile connection, the transmitting (calling) terminal sends a calling (CNG) signal (a 1100 Hz calling tone) and the receiving (called) terminal replies with a called (CED) signal (a 2100 Hz called station identifier) and a digital identification signal (DIS) at 462 Hz. After the handshaking, which consists of a transmit digital command signal (DCS), a training signal and a received confirmation to receive (CFR), the calling facsimile starts transmitting the first page with compression according to CCITT (ITU) T.4. The called facsimile is silent at this time.
Systems such as Magellan Passport allows the carriage of synchronous digital data (voice and non-voice) over a cell-based Magellan Passport network. Magellan and Passport are trademarks of Northern Telecom.
Two techniques for compression or bandwidth saving are used for the data voice component in Passport: Adaptive Differential Pulse Code Modulation (ADPCM) and silence suppression. ADPCM compression is a means of looking at successive data and coding the difference instead of the data itself. Thus the number of bits representing a sample data can go from 8 to 4, 3 or 2 bits per sample. On the other hand, silence suppression uses knowledge of speech properties and dynamics to reduce transmission requirements.